Androidは、表示されなくなったときに非同期読み込みリスト要素を停止します

このAndroid開発者トレーニングレッスンのサンプルコードで、私の質問に対する答えを見つけました。

ImageWorker.javaで、バックグラウンドタスクを起動するメソッドを見つけることができます。thadは画像をロードします。

/**
 * Load an image specified by the data parameter into an ImageView (override
 * {@link ImageWorker#processBitmap(Object)} to define the processing logic). A memory and
 * disk cache will be used if an {@link ImageCache} has been added using
 * {@link ImageWorker#addImageCache(android.support.v4.app.FragmentManager, ImageCache.ImageCacheParams)}. If the
 * image is found in the memory cache, it is set immediately, otherwise an {@link AsyncTask}
 * will be created to asynchronously load the bitmap.
 *
 * @param data The URL of the image to download.
 * @param imageView The ImageView to bind the downloaded image to.
 */
public void loadImage(Object data, ImageView imageView) {
    if (data == null) {
        return;
    }

    BitmapDrawable value = null;

    if (mImageCache != null) {
        value = mImageCache.getBitmapFromMemCache(String.valueOf(data));
    }

    if (value != null) {
        // Bitmap found in memory cache
        imageView.setImageDrawable(value);
    } else if (cancelPotentialWork(data, imageView)) {
        //BEGIN_INCLUDE(execute_background_task)
        final BitmapWorkerTask task = new BitmapWorkerTask(data, imageView);
        final AsyncDrawable asyncDrawable =
                new AsyncDrawable(mResources, mLoadingBitmap, task);
        imageView.setImageDrawable(asyncDrawable);

        // NOTE: This uses a custom version of AsyncTask that has been pulled from the
        // framework and slightly modified. Refer to the docs at the top of the class
        // for more info on what was changed.
        task.executeOnExecutor(AsyncTask.DUAL_THREAD_EXECUTOR);
        //END_INCLUDE(execute_background_task)
    }
}

AsyncTaskインスタンスへの参照は、AsyncDrawableクラスに保存されます。

/**
 * A custom Drawable that will be attached to the imageView while the work is in progress.
 * Contains a reference to the actual worker task, so that it can be stopped if a new binding is
 * required, and makes sure that only the last started worker process can bind its result,
 * independently of the finish order.
 */
private static class AsyncDrawable extends BitmapDrawable {
    private final WeakReference<BitmapWorkerTask> bitmapWorkerTaskReference;

    public AsyncDrawable(Resources res, Bitmap bitmap, BitmapWorkerTask bitmapWorkerTask) {
        super(res, bitmap);
        bitmapWorkerTaskReference =
            new WeakReference<BitmapWorkerTask>(bitmapWorkerTask);
    }

    public BitmapWorkerTask getBitmapWorkerTask() {
        return bitmapWorkerTaskReference.get();
    }
}

バックグラウンドアクティビティの最後に、AsyncTaskは、更新する必要のあるビューに最後に「アタッチ」されているかどうかを確認し、他のタスクがビューに「アタッチ」されていない場合にのみ更新を実行します。

    /**
     * Returns the ImageView associated with this task as long as the ImageView's task still
     * points to this task as well. Returns null otherwise.
     */
    private ImageView getAttachedImageView() {
        final ImageView imageView = imageViewReference.get();
        final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView);

        if (this == bitmapWorkerTask) {
            return imageView;
        }

        return null;
    }


   /**
     * Once the image is processed, associates it to the imageView
     */
    @Override
    protected void onPostExecute(BitmapDrawable value) {
        //BEGIN_INCLUDE(complete_background_work)
        // if cancel was called on this task or the "exit early" flag is set then we're done
        if (isCancelled() || mExitTasksEarly) {
            value = null;
        }

        final ImageView imageView = getAttachedImageView();
        if (value != null && imageView != null) {
            if (BuildConfig.DEBUG) {
                Log.d(TAG, "onPostExecute - setting bitmap");
            }
            setImageDrawable(imageView, value);
        }
        //END_INCLUDE(complete_background_work)
    }